This invention relates to frothed rubber latex sponge. More particularly this invention relates to blending synthetic and natural latices with a high styrene resin latex augmentation to produce foamed sponges having densities below about 16 pounds and preferably below 8 pounds per cubic foot. This invention also relates to a process for producing frothed sponge rubber at very low densities yet retaining compressive deflection and compressive set so that the product is suitable in such sponge usages as upholstery cushions, shoe insoles, carpet backing or underlayment and the like.
Froth sponge rubber, broadly speaking, has been produced by converting latex into a foam, shaping the foam, coagulating and curing the latex in the foam as by the well known No-Gel system of the literature to form a highly useful material of commerce.
An important property of froth sponged rubber for such usages is its ability to resist compression load to the proper degree. This is measured as the amount of load necessary to produce a 25% (of the original thickness) deflection or compression in the latex sponge product. This property is developed into a standard method of measurement and values for particular uses under American Society for Testing and Materials (ASTM) Standard No. D1056 and Canadian Government Specifications Board (CGSB) Standard 20GP23a. In general, to achieve a high compression resistant sponge, the density of the sponge is kept high by beating less air into the latex; and in low compression resistant sponge density is kept low by beating more air into the latex. Since for high density more rubber is used in the high compression resistant sponge, its cost is correspondingly higher. Thus, sponge rubber may be sold on a compressive deflection/weight basis. It would be highly desirable to develop a sponge material which has the same compressive deflection value as conventional sponges, but produced with a lesser amount of rubber latex. One such thought on doing this has been to blend the natural rubber with a high styrene latex such as taught in Canadian Pat. No. 537,910. In practice, however, the industry for cushions and undercushions has not appeared to go beyond about a 5% by weight addition or augmentation with high styrene latex because the high styrene latex above 5% by weight addition is known to create surface imperfections such as surface cracking and a "crows feet" appearance, and kills certain other properties of the blended latex sponge most importantly the ability of the blend to resist compressive set.
Resistance to compressive set is the ability of the sponge to not take a permanent set after application of a specific compression and release of the compression. This is a characteristic of the sponge to retain its liveliness or spring-back after a specific compression of the sponge. In essence, after a standard compression has been applied under standard conditions and then released, the sponge returns only part way back to its original shape. This property also has been made the subject of a standard under ASTM Standard No. D1056 and CGSB Standard 20GP23a. Again, also, natural rubber is known for its resistance to compressive set, and augmentation with synthetic resin latices generally directly proportionally killed the compressive set resistance. Again, it would be highly desirable to be able to develop a blended sponge material which has the same compressive set values as conventional sponge, but produced with a lesser amount of rubber latex.
Canadian Pat. No. 537,910 establishes that it is possible to blend natural rubber with synthetic resin latices with sponging of the resulting blend to obtain low foam densities, though the natural rubber latex has to be matured for 16 hours before the resin latex addition. More particularly, the patent discloses a foamable latex mixture of natural rubber and a styrene-butadiene resin latex having as much as 95% styrene, wherein the resin latex may be present in amounts up to about 30% and the resulting foams will have densities around 8 pounds per cubic foot. Such material as taught by that patent does obtain desirable compressive deflection properties; however, the high amount of styrene augmentation has completely killed compressive set characteristics as to ASTM specifications and Canadian government specifications for cushions and the like. While it is known that generally various synthetic elastomeric latices may be blended with natural rubber for producing latex sponge rubber, these generally produced products are not suitable for undercushion usage because of compressive deflection and compressive set characteristics which are only obtained at high densities, around 12 pounds per cubic foot or higher. Trials to produce densities below 10 pounds per cubic foot in styrene butadiene latices, even with high styrene augmentation in amounts of only 5-10 dry parts per 100 dry parts of elastomeric latex by weight, produced products deficient in physical properties such as compressive sets. It is believed by those skilled in the art that the addition of more than 5 dry parts of high styrene latex per 100 dry parts of elastomeric latex would make it necessary to keep densities in the 12-20 pounds per cubic foot range, dependent upon the use of the product, to maintain acceptable compressive set values and surface appearance because of the greater stiffness introduced by the high styrene. That is, generally the industry accepts that densities must be at least between 15-20 pounds for pillow cushioning, 16-18 pounds for carpet backing and 12-14 pounds for undercushion of acceptable compressive set values for example.
Also, for example, U.S. Pat. No. 3,573,246 teaches a process of modifying natural rubber with a synthetic reinforcing latex in which the blend is homogenized to achieve the blending for latex foam goods. This operation must be done under pressure and temperature conditions by the manufacturer as part of the polymerization process. The manufacturer of foam undercushion goods has neither the equipment nor the expertise to get into the polymerization business and is confined to using available commercially polymerized materials in his processes. In somewhat similar fashion, British Pat. No. 1,383,642 uses hydrazine and hydrogen peroxide to produce the requirements of low density and good deflection in a modified polymer dispersion. This patent does not set forth any mention of effect on compressive set. Again, this procedure is more appropriate for the manufacture of the latices and not for the manufacture of the sponge rubber articles.